10U FUNCTIONS OF THE NERVOUS SYSTEM. 



tral organs : or it may be excited by an impression conveyed to them through 

 afferent nerves; but in both cases its law is the same. 



IV. When the whole trunk of a motor nerve is irritated, all the muscles 

 which it supplies are caused to contract ; but when only a part of the trunk or 

 a branch is irritated, the contraction is confined to the muscles which receive 

 their nervous fibres from it. This contraction evidently results from the simi- 

 larity between the effect of an artificial stimulus applied to the trunk in its 

 course, and that of the change in the central organs by which the vis nervosa 

 is ordinarily propagated. In this instance, as in the other, there is no lateral 

 communication between the fibrils. 



VI. Comparative Anatomy and Physiology of the Nervous System. 



127. Although the structure and distribution of the Nervous System in the 

 different classes of Animals have been, until recently, but little appealed to in 

 the determi^^Hp- of its functions, they are capable of supplying evidence 

 regarding some of these, not less important in its character than that which 

 Comparative Anatomy affords to other departments of Physiology. Some 

 of the principal of these contributions will now be pointed out 



128. In the lowest tribes of the RADIATED division of the animal kingdom, 

 no nervous system has yet been discovered. These have, therefore, been 

 separated by some naturalists into a new primary group, to which the desig- 

 nation of Jlcrita has been given, on account of the (supposed) " indistinct, 

 diffused, or molecular character of their nervous system." This idea of a 

 " diffused nervous system" seems to be regarded l>y many physiologists as 

 well as naturalists as the necessary alternative, resulting from the want of 

 any definite indications of its presence. It may be said, however, to be based 

 on very erroneous notions, as to the true offices of the nervous apparatus. Its 

 influence is not required to endow the tissues with contractility ; a property 

 possessed in a high degree by the structures of many Plants, to which these 

 beings present a much greater general resemblance than they bear to the 

 higher Animals; and, even in the latter (as will be shown hereafter), this pro- 

 perty is independent of "nervous agency," although generally called into ex- 

 ercise by it. That a nervous system is not required by them for the performance 

 of the functions of Nutrition and Reproduction, otherwise than to supply, by 

 its locomotive actions, the conditions of those functions, would also appear from 

 its absence in Plants. It is on the sensible movements of these beings that 

 our belief in their possession of a nervous system must be founded, when we 

 cannot render it cognizable by our senses. But we must be careful not to draw 

 hasty inferences from such phenomena. Sensible movements are, as we have 

 seen, performed by the Dionsea and Sensitive plant, in respondence to external 

 stimuli acting on distant organs; and here the channel of communication is 

 probably the vascular system. We observe, however, that even in Polypes, an 

 impression made upon one part (one of the tentacula, for example), is propa- 

 gated to distant parts, and excites respondent movements in them, more rapid- 

 ly than we could imagine to occur, without such a channel of communication 

 as a nervous system only is known to afford. Moreover, some of their actions 

 appear to show a certain degree of voluntary power, and, therefore, of con- 

 sciousness ; being independent, so far as can be ascertained, of the operation 

 of external stimuli. These phenomena, then, would lead us to suspect the 

 existence of a nervous system in the beings which exhibit them ; not, how- 

 ever, in a " diffused" condition, but in the form* of connected filaments. For, 

 what consentanepusness of-actio can he jb&le fpr*in a being whose nervous 

 matter is incorporated -lii th<> state pf," isolated gltfbules with its tissues ? How 

 should an impression tnatle' on one part be propagated by these to a distance ? 



